Prof. Bonney — Some Notes on Gneiss. 121 



of water. Of course cases can be found where a gneiss is imitated, 

 such as the so-called Carboniferous gneiss of Guttannen.' Here, 

 however, the rock has not been restored, in any proper sense of the 

 word, to a crystalline condition ; it is only a somewhat consolidated 

 " arkose " like the Torridon sandstone, which also has been slightly 

 modified by subsequent pressure. Hence it has no more claim to 

 the name of gneiss than certain very remai"kable specimens of the 

 Torridon rock have to that of granite. Pebbles of this rock (or of 

 one exactly like it) sometimes so closely resemble granite that I have 

 found it necessary to examine them under the microscope before 

 I could feel satisfied as to their clastic character. In such cases as 

 these Nature has been perpetrating a forgery and setting a trap for 

 unwary geologists. 



Still, notwithstanding these difficulties, I think it would be rash, 

 in the present state of our knowledge, to deny the possibility of 

 gneiss being produced by the alteration of a sedimentary rock. Among 

 certain quartz-schists (not quartzites) I have occasionally found 

 interstratified bands or beds, including grains of felspar, for which 

 it seemed as reasonable to infer a clastic origin as for the other 

 constituents — that is to say, they were probably enlargements of 

 a fragment of felspar. This mineral occasionally is sufficiently 

 abundant to make the rock petrographically a quartzose gneiss. 

 With direct evidence such as this, and the indirect evidence afforded 

 by the formation of secondary felspar in certain cases of contact- 

 raetamorphism and in some crystalline schists, it would be unwise 

 to deny the possibility of gneiss having been formed by the meta- 

 morphism of a sedimentary rock. Still, I think this origin will 

 prove to be a rather exceptional one, and the masses produced 

 comparatively thin and limited in extent. 



We conclude, therefore, that the term gneiss covers a group of 

 rocks rather different in character and very different in history. 

 One (a common type) is a gneiss in consequence of an original 

 structure, and remains very nearly in its original condition. 

 Another (ahso common) owes its structure to pressure acting on a 

 rock which had already solidified and had become crystalline. A 

 third (probably rather rare and exceptional) is the result of the 

 metamorphism of materials which were originally clastic. It is 

 very probable that each of these groups will be found to possess 

 distinctive structures : to some of them I have already invited 

 attention ; others will doubtless be discovered during the next few 

 years. The enquiry is as promising as it is interesting and success 

 is probable, if the investigators remember that work in the field 

 must go hand in hand with the study of structures under the 

 microscope. These methods, when united, may be fertile, but when 

 dissociated they can only result in a false conception and a bringing 

 forth of wind. 



1 Quart. Journ. Geol. Soc. 1892, p. 390. 



